1. Technical Field of the Invention
The present invention generally relates to methods and systems for generating parametric test data for electrical devices. More particularly, and not by way of any limitation, the present invention is directed to method and system for generating a shmoo plot representative of such parametric data.
2. Description of Related Art
It is often necessary to characterize the performance of an electrical device with respect to certain operational parameters of the device. This characterization may be referred to as xe2x80x9cparametric testingxe2x80x9d. One method by which to measure the performance of a device is on a pass/fail basis. A complete parametric test would provide a pass/fail result for every possible combination of all values of the operational parameters in question. This type of test provides users of the device information as to how the device will perform over a broad range of test conditions.
A shmoo plot is a graphical representation of the ability of an electrical device to operate properly in response to various combinations of values of two variable operating parameters, or xe2x80x9ctest conditionsxe2x80x9d. For example, an integrated circuit (xe2x80x9cICxe2x80x9d) device might be repeatedly tested using different combinations of supply voltage and CPU clock signal frequency to determine the various test conditions in which the IC operates properly.
FIG. 1 illustrates a conventional shmoo plot 100. Each of the X- and Y-axes, designated 102(X) and 102(Y), respectively, represents the value of a test operating parameter. Continuing with the IC example set forth above, the X-axis 102(X) represents the value of supply voltage (Vcc) in units of volts (xe2x80x9cVxe2x80x9d) and the Y-axis 102(Y) represents CPU clock signal frequency in units of megahertz (xe2x80x9cMhzxe2x80x9d). To generate sufficient data to produce a useful shmoo plot, a device must be tested at an adequate number of combinations of X and Y operating parameter values within a range of interest bounded by Xmin, Xmax, Ymin, and Ymax, with some predetermined resolution in the X and Y interval size. Again, returning to the IC example, as illustrated in FIG. 1, Xmin and Xmax are 1.0 V and 2.1 V, respectively, while Ymin and Ymax are 470 Mhz and 1100 Mhz, respectively. The X and Y interval sizes are 0.1 V and 30 Mhz, respectively.
A xe2x80x9cpassxe2x80x9d symbol, represented in FIG. 1 by the symbol xe2x80x9c - - - xe2x80x9d, is plotted when the device passes a test performed under the combination of operating parameters identified by the corresponding Cartesian (X, Y) coordinate pair. Similarly, a xe2x80x9cfailxe2x80x9d symbol, represented in FIG. 1 by the symbol xe2x80x9cxxxxe2x80x9d, is plotted when the device fails a test performed under the combination of operating parameters identified by the corresponding Cartesian (X, Y) coordinate pair. In the IC example used herein, the IC is deemed to have xe2x80x9cpassedxe2x80x9d if it functions correctly under a given set of test conditions.
As evidenced by the shmoo plot 100, shmoo plots provide a clear depiction of the operational limits of a device under various test conditions. A passing region 104 comprises the collection of passing points. A failing region 106 comprises the collection of failing points. An observer can easily note where the performance of the device transitions from the passing region 104 to the failing region 106 as the operating parameters are varied. It will be recognized, however, that a significant amount of time is required to perform the tests that form each of the individual array elements of a shmoo plot. In a simple 16 xc3x9716 shmoo plot, assuming each test requires 100 xcexcs to perform, the entire shmoo plot would require 16xc3x9716xc3x97100 xcexcs, or 25.6 ms, to complete. Realistically, each operating parameter will be swept over a range of 100 values and each individual test could take on the order of a second to complete. Accordingly, a shmoo plot corresponding to such a parametric test would take nearly three hours to complete.
Accordingly, the present invention advantageously provides a system and method for more rapidly generating a shmoo plot by varying the resolution thereof.
One embodiment is a method of testing an electrical device to determine a range of combinations of values of N parameters for which the device functions properly. The method comprises subdividing a plot region comprising a plurality of operating points each corresponding to a particular combination of values of the N parameters into at least two partially overlapping sub-regions; testing the electrical device using the combination of values of the N variable parameters corresponding to the operating points located in each corner of each sub-region; and for each sub-region with respect to which the device does not function in the same manner at each corner operating point thereof, repeating the incrementing, subdividing and testing.